1. Field of the Invention
The present invention relates to a thermal image transfer recording medium comprising a support, a thermosensitive image transfer layer formed thereon and a back layer with a thickness of 0.001 to 0.1 .mu.m, on the back side of the support, opposite to the thermosensitive image transfer layer, which recording medium is excellent in the transport performance and is capable of producing high quality images.
2. Discussion of Background
Recently a thermosensitive image transfer recording system using a thermal head is widely used because of the advantages that it is noiseless, the apparatus for use in the system is relatively inexpensive and can be made small in size, the maintenance is easy, and printed images are stable in quality.
As a thermosensitive image transfer recording medium for use with the above-mentioned thermosensitive image transfer recording system, for example, a thermosensitive image transfer recording sheet comprising a support such as a polyester film and a thermosensitive image transfer layer (a thermofusible ink layer) formed thereon which comprises a wax such as a natural wax or synthetic wax, a coloring agent, and a thermoplastic resin is employed.
Such a conventional thermosensitive image transfer recording medium has the disadvantages that the transport performance is unsatisfactory and the heat resistance of a surface thereof in contact with a thermal head is low.
In order to remove the above disadvantages, there are proposed a variety of thermal image transfer recording media having a back layer comprising materials with excellent lubricating properties and heat resistance, which is provided at the back side of the support. For example, a thermal image transfer recording medium comprising a heat-resistant protective film with a thickness of 0.5 to 5 .mu.m, serving as a back layer which is prepared by coating a silicone resin on the back side of a support and drying it at temperatures of 150.degree. to 200.degree. C., is disclosed in Japanese Patent Publication 58-13359; a thermal image transfer recording medium comprising a back layer which comprises a wax or oil, is disclosed in Japanese Laid-Open Patent Application 59-148697; a thermal image transfer recording medium comprising a back layer which is a thin film of a silicone rubber is disclosed in Japanese Laid-Open Patent Applications 62-33682 and 62-109688; and a thermal image transfer recording medium comprising a back layer made of a condensation-type silicone rubber is disclosed in Japanese Laid-Open Patent Application 1-141789. In addition, there is proposed a method of supplying a thermal head or the back side of a thermal image transfer recording medium with oil in the course of printing operation, as disclosed in Japanese Laid-Open Patent Application 59-196293.
Although, in such conventional thermal image transfer recording media, the lubricating properties and the heat resistance are improved to a certain degree, they are still unsatisfactory for practical use. For example, a silicone resin is suitable for a heat-resistant protective film because a heating loss thereof is as small as about 5% up to temperatures of 550.degree. C. However, when back layer comprising a silicone resin with a high heat resistance is provided on the back side of the support in order to prevent the recording medium from sticking to a thermal head, the thickness of the back layer is necessarily increased to 0.5 .mu.m or more, as disclosed in Japanese Patent Publication 58-13359. In the case where the thickness of the back layer is thin, the heat resistance thereof is lowered. On the other hand, the thickness of the back layer becomes thick, the sensitivity and the lubricating properties thereof deteriorate.
In the prevailing high-speed thermal printer, for the purpose of improving the image quality, the thermal image transfer recording medium is caused to pass between a hard platen roll and a thermal head, with the pressure applied to the recording medium by the thermal head increased, so that the transport performance of the thermal image transfer recording medium becomes poor and the above-mentioned conventional back layer easily sticks to a thermal head. Moreover, when high energy is applied to the image transfer recording medium from the thermal head, the thermal image transfer recording medium is cut off. Further, in the case where the back layer comprises a silicone resin with a high crosslinking density for the purpose of imparting the high heat resistance to the back layer, the back layer becomes so hard that the thermal head is easily abraded when the recording medium is repeatedly caused to pass through the thermal head.
Moreover, to obtain a silicone resin with a high cross linking density for use in the back layer, it is necessary to heat the resin at high temperatures for a long time. This induces the contraction of the support film, and consequently, the obtained image quality is degraded or the productivity of the recording medium is lowered.
The conventional thermal image transfer recording medium comprising the back layer made of a wax or oil has the lubricating properties. However, when this type of recording medium is rolled, the low-molecular-weight material of the oil or wax contained in the back layer readily transfers to the adjacent ink layer. As a result, the lubricating properties of the back layer are lowered and the image fixing properties are decreased.
With respect to the method of supplying a thermal head or a back side of the recording medium with oil in the course of the printing operation, it is necessary to equip a printer with an oil feeding apparatus, even though the previously mentioned shortcomings are overcome.
The conventional thermal image transfer recording medium having a back layer of a silicone resin thin film does not have sufficient lubricating properties.
In addition, the conventional thermal image transfer recording medium having a back layer comprising a condensation-type silicone rubber has the shortcoming that it is necessary to apply the thermal energy thereto to dry the water generated from the silicone rubber.